1. Field of the Invention
The present invention relates to a cell process in an asynchronous transfer mode (ATM) switch.
2. Background of the Related Art
Multicast is a bidirectional communication among multiple sites, and refers to the type of communication that is performed between one transmission part and multiple destination parts. For instance, transmitting an e-mail to all addresses stored in an address book is multicasting. Multicasting is also the type of communication used for a teleconference such as a video-conference or audio-conference where one transmission part transmits a video or audio signal to multiple selected destination parts, i.e., sites, or for the transmission of video data from a video-on-demand (VOD) server to multiple subscriber set top boxes.
Unicast is in contrast to the multicast, and means the communication type that is performed between one transmission part and one destination part.
As related art multicasting systems are used in ATM switching, there exist a cell-copy type multicasting system and an address-copy type multicasting system.
FIG. 1 is a block diagram illustrating the construction of a related art cell-copy type multicasting system. According to this system, a cell-copy circuit 10 is provided in front of a switch fabric, and a multicast cell having various destination addresses is copied to various unicast cells having one destination address. In the cell-copy type multicasting, the input multicast cell at an input terminal is copied to a plurality of unicast cells corresponding to respective output ports, i.e., fan-out ports, and the copied unicast cells are stored in a plurality of cell memory spaces in a shared buffer 30. The buffer number and addresses are stored in an address queue 20.
In the above-described cell-copy type multicasting structure, the number of cells stored in the shared buffer 30 is increased in proportion to the number of destinations of the multicast cells or the number of fan-out ports. This causes the cell traffic to be increased in the ATM switch and leads to excessive cell loss in the multicast environment.
FIG. 2 is a block diagram illustrating the construction of a related art address-copy type multicasting system. According to this address-copy type system, a multicast cell is stored in a shared buffer 60, and the address of the shared buffer 60 that stores the multicast cell is copied by an address copy circuit 40 and then stored in an address queue 50 corresponding to respective fan-out ports.
In the above-described address-copy type multicasting structure, the multicast cell is stored in a cell memory space in the shared buffer 60, and the buffer number and the addresses of the buffer are copied by the address copy circuit 40 and then stored in the address queue 50. In this case, the effective traffic is not increased in the shared buffer 60, but the traffic of the address queue 50 that stores the addresses of the multicast cells is increased, causing the head-of-line (HOL) blocking phenomenon also to be increased. This blocking phenomenon in the multicast environment results in the deterioration of throughput of the ATM switch.
FIG. 3 illustrates HOL blocking in a related art shared buffer memory type ATM switch. It is assumed that cells A, B, C, and D positioned on the heads of respective address queues should be outputted to a cell time slot. Since the cells B, A, and C are stored in the third buffer of a shared buffer memory 70 together, they cannot be simultaneously read out by the cell time slot. That is, the shared buffer memory 70 can perform the read or write operation for only one cell of cells B, A and C at a time.
Accordingly, only one cell among the cells B, A, and C is read out by the third buffer, and the remaining two fan-out ports are in an idle state, which deteriorates the throughput of the whole switching system. According to the queuing theory, it is known that the throughput deteriorates by about 46%.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.